1. Field of the Invention
The present invention relates to a process for removing silicon-containing impurities from water especially industrial water.
Water is commonly used in industrial systems, eg heating or cooling systems. Silica is present in water obtained from natural sources, and is therefore present in industrial water. If the concentration of silica exceeds a threshold value which depends upon the conditions, but is often around 150 mgl.sup.-1, precipitation occurs, and fouling of surfaces by scale can take place. Formation of scale reduces efficiency of, for example, heat exchangers, it impedes flow, and it is costly and difficult to remove.
Scale formation on surfaces in cooling waters can arise in systems where pure water is lost by evaporation (for example, in cooling towers), and is replaced from another source (known as make up water). To reduce the consumption of water, the water is recirculated around the cooling system, and, ideally, the only water added is to replace that lost as steam from cooling towers. The concentration of silica in the water lost is low relative to that in the make up water, so the concentration of silica increases in the circulating water, eventually to the point at which precipitation and scaling occur. To prevent this, some water is removed from the circulation system (blow down) and is replaced by augmenting the make up water.
Any increased requirement in make up water is undesirable in terms of cost and, particularly in arid areas, availability. Alternative methods of preventing scale formation are desirable to lessen the need for increased make up.
Scale inhibiting chemical treatments are used to allow some minerals, such as calcium carbonate, to be present in solution at increased concentrations, but similar treatments for silica are comparatively ineffective. In addition, the discharge of chemicals into the environment is becoming increasingly undesirable. An alternative method of scale prevention is the removal of the impurities from the water so as to reduce the concentration. Examples of these prior art processes are as follows.
2. Description of the Prior Art
One known process for removing silicon-containing, and other, impurities from industrial water is the lime-soda softening process. This is described by F DeSilva in Chemical Engineering, August 1996, as being a sidestream process whereby scale forming ions are removed from cooling water systems. A portion of the recirculating water is treated with lime or caustic, soda ash and magnesium oxide to precipitate respectively carbonate hardness, non-carbonate hardness, and silica. The sludge formed may be dewatered.
Evaporation may also be used for reducing the concentration of scale forming minerals. A vapour-compression process, as described by B P Boffardi and A L Smith, Chemical Engineering, July 1995, is used to return purified water to the system. However, the nature of the evaporation process renders the equipment susceptible to scaling and fouling, and so maintenance costs are high.
Colloidal alumina may be used to adsorb silica from blow down water, as stated in U.S. Pat. No. 5,512,181. Dialysing the blow down produces a permeate with a reduced silica content.
Chemical treatments are used to increase the concentrations of scale forming minerals which may be present without deposition occurring. The minerals commonly treated include calcium carbonate, calcium sulphate, calcium phosphate and silica. Effective treatments for calcium salts exist. Calcium carbonate may be controlled by the use of polyphosphates, phosphonates and some polymers (eg polyacrylates, as described in U.S. Pat. No. 4,168,230). Calcium sulphate may be controlled with phosphates and aminomethylene-based polymers. Calcium phosphate may be controlled using certain polymers. Chemical treatments for the prevention of silica scale exist, for example polymaleic acid polymer, and carboxylate-sulphonate polymers (W M Hann, S T Robertson, J H Bardsley, 54.sup.th International Water Conference, Pittsburgh, Pa., Oct. 11-13, 1993), but are less effective than those for calcium salts.
Prior art references GB-A-2,235,440 and U.S. Pat. No. 3,887,462 describe the use of solid agents including Portland cement to treat waste containing waters. These references do not disclose specifically treatment of waters containing colloidal silica.
U.S. Pat. No. 4,462,905 describes the removal of amorphous silica from geothermal waters using solid particles but does not envisage the new method described herein.